This invention relates to a process for removing olefin oxides, such as ethylene oxide and propylene oxide, from gas compositions containing such, and more particularly to a process for removing ethylene oxide in exhaust gases discharged from ethylene oxide sterilizers.
Ethylene oxide sterilizers are typically utilized, for example, in hospitals and medical product manufacturing facilities for treating articles such as plastic, paper, rubber or the like which cannot withstand heat sterilization. Sterilization is effected when ethylene oxide reacts with contaminating microorganisms to kill or inactivate them.
The sterilization process is typically carried out in a chamber which is evacuated after the articles to be treated are placed within. Either one-hundred percent ethylene oxide or an admixture with an inert diluent such as dichlorodifluoromethane or carbon dioxide is then introduced to the chamber and allowed to remain therein until the sterilization process is completed, usually from about 1 to 8 hours. After the sterilization period is completed, the gas in the chamber is exhausted and the sterilized articles removed. In order to ensure safe worker exposure levels, it may be necessary to exhaust the chamber in a series of post-evacuations prior to entry into the chamber. Ethylene oxide may be discharged to the environment with each post-evacuation, although concentrations will decrease with each successive evacuation.
A number of evacuations are needed because the product may absorb relatively substantial amount of the olefin oxide, which off-gasses from the product only gradually. At the end of post-evacuations, the chamber may be ventilated with relatively high flow rates of air. This air containing small amounts of off-gassed olefin oxide is ordinarily released into the environment. Even after the sterilized product is removed from the chamber, additional amounts of olefin oxide may be released into the work place because of off-gassing from the product. Because olefin oxide is off-gassed or diffuses out of the product with time, this is the major reason that sterilization plants quarantine the products, preferably in an isolated area after exposure to olefin oxide. The quarantined areas are ordinarily heavily ventilated to reduce worker exposure to the fumes. The small amounts of olefin oxide which are caught-up in the ventilation system are ordinarily released into the environment.
Due to the highly toxic nature of ethylene oxide, special precautions must be taken to assure proper ventilation to the area. Generally, ventilation systems are provided to direct ethylene oxide outside of the building. However, such systems are not entirely satisfactory from an environmental protection standpoint, particularly because there is still the possibility that human receptors outside of the building may unknowingly come into contact with the ethylene oxide before it has been sufficiently dispersed in the ambient atmosphere. Moreover, unfavorable meteorological conditions can contribute to unacceptably high ground level ambient ethylene oxide concentrations and/or hazardous in-plant worker exposure levels due to ethylene oxide recirculation back into the building. Such concern becomes paramount with the acceptable ethylene oxide worker exposure level reduced below one part per million. Consequently, a means must be provided to control the venting of ethylene oxide, particularly in any areas where human receptors can be exposed to such releases.
In view of increased concern over the environment, various methods have been devised by the art in an attempt to remove toxic olefin oxides, such as ethylene oxide and propylene oxide, from exhaust gases containing such. For example, U.S. Pat. No. 3,851,043 to Gunther discloses a method of treating exhaust gases from ethylene oxide sterilizers by passing the exhaust gas through a column containing a strong acid, cationic active ion exchange resin in the hydrogen form. The Gunther patent also discloses the use of an oxidizing resin manganese green sand in place of the ion exchange resin. The process taught by Gunther is a single step, batch process, having the disadvantage that the absorption beds must be either regenerated or disposed of once they have become saturated.
It is well-known in the art that olefin oxides can be hydrolyzed to alkylene glycols, which in dilute solutions are not considered to be environmentally hazardous. These acid catalyzed reactions are described by Morrison and Boyd, Organic Chemistry, Second Edition, pp. 887ff, 1966, and U.S. Pat. Nos. 1,641,710, 2,636,906 and 4,165,440.
U.S. Pat. No. 4,112,054 to Feingold et al discloses a method of removing ethylene oxide from gaseous mixtures by passing the gas through an aqueous acidic solution to simultaneously dissolve the ethylene oxide and convert the ethylene oxide to ethylene glycol and other products at a pH of less than 4 and an initial temperature of 50 degrees C. The Feingold et al patent is directed toward relatively small hospital sterilizers, and particularly toward a sterilizing means which can be rendered portable, to afford quick and efficient use. In addition, the Feingold et al process is disclosed as having an efficiency of 97% using acids such as oxalic acid, sodium bisulfate, hydrochloric acid, and hydrobromic acid. Of the acids described by Feingold et al as being most efficient, halogenated acids will produce halohydrin by-products, i.e., ethylene bromo-and chlorohydrins, which are considered highly toxic substances, and oxalic acid is extremely expensive and not cost effective. In addition, in their process, Feingold et al absorb and convert the olefin in one step. Therefore, the absorption of ethylene oxide cannot be optimized, and the conversion to alkylene glycols likewise cannot be optimized since the exothermic heat of solution using an acidic solution to absorb the olefin oxide reduces the dissolution rate. The Feingold et al patent is also a batch process and limited in application to sterilizers which do not use steam ejectors for evacuation.
U.S. Pat. No. 2,135,271 to Balcar is directed toward a method of optimizing glycol production. Disclosed is a cyclic method wherein an olefin oxide is absorbed and converted via thermal hydrolysis to an alkylene glycol, an the alkylene glycol solution is then recycled back to the absorber to absorb more olefin oxide until the concentration of the alkylene glycol reaches a point where it is commercially economical to separate the glycol from solution. The Balcar patent is not concerned with the efficiency of removing olefin oxides from gaseous mixtures.
Accordingly, it is an object of this invention to provide a highly efficient process for removing olefin oxides from gas mixtures containing olefin oxides in order that environmental hazards may be greatly reduced.
A further object of the present invention is to provide for the removal of olefin oxides from gas compositions in a process useable on an industrial scale.
A further object of tis invention is to remove at ambient pressure more than about 99% of olefin oxides from a gas composition containing such.
A further object of the invention is to provide a process for converting more than about 99% of the olefin oxides into substances which are not environmentally dangerous.
A further object of the invention is to provide a process wherein olefin oxides can be efficiently converted to alkylene glycols using sulfuric acid as a catalyst, which does not convert to toxic substances, and is relatively inexpensive to use.
A further object of the invention is to provide a process wherein olefin oxides that are "off-gassed" from the product either during chamber ventilation or in quarantine areas and released into a ventilation system can be removed from the ventilation streams.
A further object of the invention is to provide a process wherein the inert diluent for the olefin oxides is recovered.
A further object of the invention is to provide a process wherein the alkylene glycol generated by the reaction of olefin oxide with water is biodegraded into carbon dioxide and water.
Further objects of the invention will become apparent from the following description and claims.